In the UMTS (Universal Mobile Telecommunications System) Network, LTE (Long Term Evolution) has been under study for the purpose of further increasing data rate, achieving low delay and the like (for example, see Non Patent Literature 1). In LTE, as multi-access schemes, OFDMA (Orthogonal Frequency Division Multiple Access) based scheme is used for the downlink and SC-FDMA (Single Carrier Frequency Division Multiple Access)-based scheme is used for the uplink.
In the LTE system, uplink signals are mapped to appropriate radio resources and transmitted from a user terminal to a radio base station apparatus. Specifically, uplink user data is transmitted using a PUSCH (Physical Uplink Sheared CHannel). And, uplink control information (UCI) is transmitted using the PUSCH when it is transmitted together with the uplink user data, or is transmitted using a PUCCH (Physical Uplink Control CHannel) when it is transmitted alone.
Uplink control information (UCI) includes transmission acknowledgement (ACK/NACK) for a PDSCH (Physical Downlink Shared CHannel), scheduling request, CSI (Channel State Information) and so on (for example, see Non Patent Literature 2). The channel state information (hereinafter referred to as “CSI”) is information based on an instantaneous channel state of the downlink, which includes, for example, channel quality information (CQI), precoding matrix indicator (PMI), rank indicator (RI) and so on. This CSI is reported periodically or aperiodically from a user terminal to a radio base station apparatus.
Aperiodic CSI is transmitted from the user terminal to the radio base station apparatus in accordance with a trigger from the radio base station apparatus. This triggering (aperiodic CSI triggering) is included in an uplink scheduling grant (hereinafter referred to as “UL grant”) (DCI format 0/4) transmitted in a PDCCH (Physical Downlink Control CHannel). The user terminal transmits the aperiodic channel state information (hereinafter referred to as “A-CSI”) using a PUSCH designated by the UL grant in accordance with the trigger included in the UL grant. Such transmission of A-CSI is also called aperiodic CSI (CQI/PMI/RI) Reporting).
In 3GPP, in order to achieve further broadbandization and higher speeds, there has been studied a succeeding system to LTE (for example, LTE-Advanced). In the LTE-A system, it is preferable to realize a broader band while keeping backward compatibility with the LTE system. In the LTE-A system, it has been studied to aggregate a plurality of fundamental frequency blocks (CCs: component carriers) into a broad band (5 CCs are aggregated into a band of 100 MHz), where each fundamental frequency block is a band available in the LTE system (for example, 20 MHz).
In the LTE-A system, at least one cell is provided for one CC and a user terminal is configured to be able to communicate in plural cells of different CCs. In each CC, one cell where the user terminal performs communication mainly is called a serving cell. In the LTE-A system, the user terminal performs radio communication in plural serving cells of different CCs thereby to realize broadbandization of the system band.
In the LTE-A system, when the user terminal performs radio communication in plural serving cells of different CCs, communication states such as interference levels vary among serving cells. When the aperiodic CSI reporting is performed in the LTE-A system, it is preferable to flexibly report aperiodic channel state information (A-CSI) of a downlink serving cell required for the network among serving cells.